The invention relates to a circuit for compression or expansion of an electrical signal.
Companders are used to improve the signal-to-noise ratio during a signal transmission across long distances and during signal storage. The mode of operation consists of raising or emphasizing weak signals in terms of their level before transmission or storage and in lowering or de-emphasizing them after transmission or reproduction. Thus interference signals also on the transmission paths are also lowered or de-emphasized. Moreover, interference signals may be obscured by useful signals to a greater or lesser extent. This obscuring or masking effect is a physiological phenomenon and depends on how much the useful signal and the interference signal differ in frequency from each other. Therefore it is advisable to subdivide the low frequency spectrum into several frequency selective channels and to control the effective transmission factor in these channels only in dependence on the useful signals occurring therein. A masking effect which would be sufficient for professional purposes may be achieved with four frequency selective channels. For semi-professional and consumer purposes, a lower cost is desirable. Considering that the cost is approximately proportional to the number of channels the restriction to two channels is sensible for semi-professional purposes.
However, it has proved that one such compander, which was conceived according to the principles of a four-channel compander--with the exception of the other crossover frequencies--already has a noise effect in the case of certain extreme signal performance, e.g. strong spectrally pure signals in the base note range. If it were only important to process these signals free of noise then the crossover frequency of the two compander channels could be lowered. However, this results in unfavourable noise characteristics in the treble range.